1. Field of the Invention
This invention relates to a measurement-while-drilling nuclear magnetic resonance tool, and more particularly to a nuclear magnetic resonance tool having electromagnetic sensors inside a drill collar.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the xe2x80x9cbottom hole assemblyxe2x80x9d or the xe2x80x9cBHAxe2x80x9d) carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or xe2x80x9cmud motorxe2x80x9d which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies (xe2x80x9cBHAxe2x80x9d) generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) sensors. Such sensors traditionally have electro-magnetic propagation sensors for measuring the resistivity, dielectric constant, water saturation of the formation, nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other downhole sensors that have been used include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. More recently, nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) sensors have gained extreme interest as MWD sensors as such sensors can provide direct measurement for water saturation porosity and indirect measurements for permeability and other formation parameters of interest.
NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool. A radio frequency (RF) coil disposed between the magnets or around the magnets induces an RF magnetic field. The magnets and the RF coils are positioned so that the static and RF fields are perpendicular to each other at least over a portion of the formation surrounding the NMR tool wherein the static field has a substantially uniform strength. This region is the region of interest or region of investigation. The NMR measurements corresponding to such region are needed to determine the formation parameters of interest.
MWD sensors are located inside or outside of a drill collar for performing measurements on the formation and its fluid content. A conventional drill collar is a metallic structure that conveys the torque required for the drilling operation. Also, the drill collar is a conduit for the drilling fluid or mud that is used to lubricate the drill bit and carry the cuttings to the surface. Since audio and radio frequency electromagnetic fields do not penetrate the metallic body of the drill collar, sensors of electromagnetic fields must be mounted outside the metallic body of the drill collar. These sensors are subject to abrasions resulting from particles in the drilling mud and the impact of the sensor against the earth formation. In some cases, shields or protective coatings are used on the drill collar to protect the sensors. Often, wear bands are employed on the drill collar to provide an adequate amount of standoff between the sensors and the formation thereby reducing or eliminating the impact of the sensor against the earth formation.
The present invention provides for a pulsed NMR tool housed in a drill collar wherein the sensors are located on the inside of the drill collar without interference to the audio and radio frequency electromagnetic fields by the collar thus protecting the sensors without the use of additional shields or wear bands.
The disadvantages of the prior art are overcome by the apparatus and method of the present invention which protects a sensor from impact and abrasion while drilling a borehole. The apparatus according to the present invention, for example, an NMR device, includes a drill collar having a section of electrically non-conductive material. The electrically non-conductive material has a rugged structural strength for conveying required drilling operation torque and load. A sensor which, for example, induces a radio frequency (RF) magnetic field, is located inside the drill collar within the section of electrically non-conductive material wherein the sensor is protected from impact and abrasion while drilling a borehole without interference to the RF field. Preferably, the electrically non-conductive material is a composite material.
The sensor is located, for example, within a load bearing section of the drill collar. Alternatively, the sensor is located inside a removable probe positioned inside a channel through a longitudinal axis of the drill collar. The channel allows a drilling fluid to flow through the drill collar. Stabilizers are used for stabilizing the probe within the channel.
Preferably, metallic tool joints are connected to the section of electrically non-conductive material for coupling the drill collar to a drilling assembly. Hardfacing can also be embedded on an outer surface of the drill collar for providing additional abrasion protection.
The NMR device further includes a magnet assembly for generating a static magnetic field in the earth formations traversed by the borehole. An electronic driving circuitry drives the sensor. The magnet assembly and the electronic driving circuitry are, preferably, located within the section of composite material of the drill collar, or, alternatively, can be located elsewhere within the drill collar.